I assume you are using two routers to give you two subnets on purpose rather than having two routers because of needing the extra ethernet ports.

The two network ports on the NAS are there to provide either 'Link Aggregation', should that be necessary or 'Fail-over and Fail-back' to keep data available should the network go down and the mirror server has to take over.

They are not there to provide combining two subnets (doing so would degrade the security found in using the two subnets).